Pegging is when someone fuck's you in the ass with a strap on.
Guys have a prostate glad deep in their bum that can be stimulated by inserting a finger, dildo, strap-on or penis.
If you are going to use a dildo on your ass, make sure it's got a flared base... which will stop it from going all the way in, and getting lost up there.
Most Domme's offer pegging in their standard repertoire, and should be able to satisfy any needs you might have.
You should also be able to get a strap on at any sex shop, they are fairly standard.